FULL PAPER 1902376 (1 of 11) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com Goosebumps-Inspired Microgel Patterns with Switchable Adhesion and Friction Bin Li,* Michael Kappl, Lu Han, Jiaxi Cui, Feng Zhou, and Aránzazu del Campo DOI: 10.1002/smll.201902376 directly links to the underlying mecha- nism of skin deformation and implies a useful function. Goosebumps represent an array of tiny bumps generated from a smooth surface through the contraction of miniature muscles that are attached to each hair, and cause the surrounding area to protrude to form bumped structures. The occurrence of goosebumps helps to regulate body temperature and takes part in heat transfer balance, but also delivers important emotional information by altering touch adhesion and friction. In the skins covered with thick hairs, goose- bumps can significantly increase moving drag by tilting up the long hairs, for example, the feather of flying birds. These phenomena suggest that the reversible morphology change of the goosebumps- like structure on a surface constitutes a novel strategy to develop “smart” coatings such as biosensors and actuation devices. Herein, we attempt to mimic the reversible goosebump surface by using responsive hydro- gels. Hydrogels can undergo topographical changes between swollen and shrunken states through switching the solvation state of the polymer network by external stimuli such as pH or temperature. [14–16] Hydrogels have been widely used to fab- ricate smart materials, like artificial muscles. [3,17] During the shrinking or swelling, water molecules are expelled from or sorbed in the polymer network, which is a typical slow diffu- sion process. Several strategies have been developed to reduce diffusion pathway to speed up the response, including intro- ducing porous structures in the hydrogel or decreasing the size of the hydrogel to increase the response speed. Microgels represent one of the most promising materials to achieve fast A substrate mimicking the surface topography and temperature sensitivity of skin goosebumps is fabricated. Close-packed arrays of thermoresponsive microgel particles undergo topographical changes in response to temperature changes between 25 and 37 °C, resembling the goosebump structure that human skin develops in response to temperature changes or other circumstances. Specifically, positively charged poly[2-(methacryloyloxy) ethyltrimethylammonium chloride] (PMETAC) brushes serve as an anchoring substrate for negatively charged poly(NIPAm-co-AA) microgels. The packing density and particle morphology can be tuned by brush layer thickness and pH of the microgel suspension. For brush layer thickness below 50 nm, particle monolayers are observed, with slightly flattened particle morphology at pH 3 and highly collapsed particles at pH above 7. Polymer brush films with thickness above 50 nm lead to the formation of particle multilayers. The temperature responsiveness of the monolayer assemblies allows reversible changes in the film morphology, which in turn affects underwater adhesion and friction at 25 and 37 °C. These results are promising for the design of new functional materials and may also serve as a model for biological structures and processes. Goosebumps [+] Present address: Physik Department, Technische Universität München, James-Franck-Straße 1, 85 748 Garching, Germany The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/smll.201902376. Dr. B. Li, [+] Dr. L. Han, Prof. J. Cui, Prof. A. del Campo INM – Leibniz Institute for New Materials Campus D2 2, 66123 Saarbrücken, Germany E-mail: Bin1.Li@tum.de Dr. M. Kappl Max Planck Institute for Polymer Research Ackermannweg 10, 55128 Mainz, Germany Dr. L. Han, Prof. J. Cui Institute of Fundamental and Frontier Sciences University of Electronic Science and Technology of China 610054 Chengdu, China Prof. F. Zhou State Key Laboratory of Solid Lubrication Lanzhou Institute of Chemical Physics Chinese Academy of Sciences 730000 Lanzhou, China Prof. A. del Campo Chemistry Department Saarland University 66123 Saarbrücken, Germany 1. Introduction Human skin, as well as some animal skin, have the ability to reversibly and rapidly change the surface morphology in response to specific external/internal stimuli in the environ- ment. [1–7] Examples include the hierarchical structures of gecko foot for clamping; adaptive deformation of dolphin skin for fast swimming, or the wrinkled geometry of earthworm cortex for friction reduction. Inspired by these biological examples, many strategies have been developed to create responsive surfaces to mimic these functions. [8–13] One important involuntary surface morphology change in human skin is the occurrence of goose- bumps, which has not been mimicked until now, although it Small 2019, 1902376